In the past, polymers, particularly urethane rubber, have been used for a variety of applications in which it is desirable that the product have some electrical conductivity.
One example involves biasable transfer members, e.g., biasable transfer rolls or webs, which are used in electrostatographic copying systems or apparatus to transfer toner images from an electrostatographic element, such as a photoconductor, to a final support material or receiver, such as a web or sheet of paper.
In electrostatography, an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrostatographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with charged toner particles. If desired, the toner image can then be transferred to a final support material or receiver such as a web or sheet of paper and affixed thereto to form a permanent record of the original.
The process of transferring toner material from the electrostatographic element or photoconductor to the receiving sheet or copy sheet, is realized at a transfer station. In a conventional transfer station, transfer is commonly achieved by applying electrostatic force fields in a transfer nip sufficient to overcome the forces which hold the toner particles to their original support surface on the photo-receptive member or photoconductor. These electrostatic force fields operate to attract and transfer the toner particles over and onto the copy sheet or other supporting second surface.
As a means of controlling the forces acting on the toner during transfer so that transfer of the toner image from the photoconductor to the final support material occurs, a biasable transfer member, such as a biasable transfer roll is utilized.
For example, Chen et al, in U.S. Pat. Nos. 4,729,925 and 4,742,941 disclose, as coating materials for biasable transfer members, polyurethane elastomers made from certain polyisocyante prepolymers and polyols in which the resistivity can be maintained between 1.0.times.10.sup.9 and 1.0.times.10.sup.11 ohm cm by copolymerizing with the polyisocyanate prepolymers and polyol hardening compounds used to make the polyurethane elastomers certain polyol charge control agents formed from certain carboxylated aromatic sulfonate salts which have been esterified with polyester diols or certain carboxylated aromatic sulfonamidosulfonyl salts esterified with polyester diols as represented by formulas (I) and (II): ##STR1## wherein R.sup.1 represents: ##STR2## R.sup.6 represents sulfonate, oxyphenylene sulfonate, oxycyclohexylene sulfonate or p-toluenesulfonamidosulfonyl;
R.sup.2 represents oxyphenylene sulfonate, oxycyclohexylene sulfonate, or p-toluenesulfonamidosulfonyl; PA1 R.sup.7 represents: ##STR3## R.sup.3 represents a straight or branched chain alkylene group having 2 to 7 carbon atoms; PA1 R.sup.4 is the same as R.sup.3 or is .paren open-st.R.sup.5 --O.paren close-st..sub.x R.sup.5 ; PA1 R.sup.5 is the same as R.sup.3 ; PA1 m and n in formula (I) are integers which together are of sufficient value to achieve an R.sup.1 molecular weight of 300 to 30,000; PA1 m and n in formula (II) are integers which together are of sufficient value to achieve an R.sup.7 molecular weight of 300 to 30,000; and PA1 M represents hydrogen, an alkali metal, ammonium or P.sup.+ (C.sub.6 H.sub.5).sub.3 CH.sub.3. PA1 e.s.a.=electrode surface area, PA1 (i) a bis[oxydiethylenebis(polycaprolactone)yl]sulfoaryldicarboxylate or a bis[oxydiethylenebis(polycaprolactone)yl]sulfoamidosulfonylaryldicarboxyla te, as represented by formula (I) or formula (II): ##STR4## wherein R.sup.1 represents: ##STR5## R.sup.6 represents sulfonate, oxyphenylene sulfonate, oxycyclohexylene sulfonate or p-toluenesulfonamidosulfonyl; PA1 R.sup.2 represents oxyphenylene sulfonate, oxycyclohexylene sulfonate, or p-toluenesulfonamidosulfonyl; R.sup.7 represents: ##STR6## R.sup.3 represents a straight or branched chain alkylene group having 2 to 7 carbon atoms; R.sup.4 is the same as R.sup.3 or is .paren open-st.R.sup.5 --O.paren close-st..sub.x R.sup.5 ; R.sup.5 is the same as R.sup.3 ; PA1 m and n in formula (I) are integers which together are of sufficient value to achieve an R.sup.1 molecular weight of 300 to 30,000; m and n in formula (II) are integers which together are of sufficient value to achieve an R.sup.7 molecular weight of 300 to 30,000; and M represents hydrogen, an alkali metal, ammonium or P.sup.+ (C.sub.6 H.sup.5).sub.3 CH.sub.3, and PA1 (ii) a complex of a ferric halide selected from the group consisting of ferric fluoride, ferric chloride and ferric bromide and ethylene glycol or an oligoethylene glycol selected from the group consisting of di-, tri-, and tetraethylene glycol PA1 (i) a bis [oxydiethylenebis (polycaprolactone)yl]sulfoaryldicarboxylate or a bis[oxydiethylenebis(polycaprolactone)yl]sulfonamidosulfonylaryldicarbox ylate, as represented by formula (I) or formula (II): ##STR7## wherein R.sup.1 represents: ##STR8## R.sup.6 represents sulfonate, oxyphenylene sulfonate, oxycyclohexylene sulfonate or p-toluenesulfonamidosulfonyl; R.sup.2 represents oxyphenylene sulfonate, oxycyclohexylene sulfonate, or p-toluenesulfonamidosulfonyl; R.sup.7 represents: ##STR9## R.sup.3 represents a straight or branched chain alkylene group having 2 to 7 carbon atoms; R.sup.4 is the same as R.sup.3 or is .paren open-st.R.sup.5 --O.paren close-st..sub.x R.sup.5 ; R.sup.5 is the same as R.sup.3 ; m and n in formula (I) are integers which together are of sufficient value to achieve an R.sup.1 molecular weight of 300 to 30,000; m and n in formula (II) are integers which together are of sufficient value to achieve an R.sup.7 molecular weight of 300 to 30,000; and M represents hydrogen, an alkali metal, ammonium or P.sup.+ (C.sub.6 H.sup.5).sub.3 CH.sub.3, and PA1 (ii) a complex of a ferric halide selected from the group consisting of ferric fluoride, ferric chloride and ferric bromide and ethylene glycol or an oligoethylene glycol selected from the group consisting of di-, tri-, and tetraethylene glycol PA1 in a molar ratio of a dicarboxylate of formula (I) or formula (II) to complex of from 1.0:5.0 to 5.0:1.0, the coating being in electrical contact with the conductive substrate such that the coating is capable of transmitting a bias potential from the substrate to the outer periphery of the coating. PA1 (i) a bis[oxydiethylenebis(polycaprolactone)yl]sulfoaryldicarboxylate or a bis[oxydiethylenebis(polycaprolactone)yl]sulfoamidosulfonylaryldicarboxyla te, as represented by formula (I) or formula (II): ##STR10## wherein R.sup.1 represents: ##STR11## R.sup.6 represents sulfonate, oxyphenylene sulfonate, oxycyclohexylene sulfonate or p-toluenesulfonamidosulfonyl; R.sup.2 represents oxyphenylene sulfonate, oxycyclohexylene sulfonate, or p-toluenesulfonamidosulfonyl; PA1 R.sup.7 represents: ##STR12## R.sup.3 represents a straight or branched chain alkylene group having 2 to 7 carbon atoms; R.sup.4 is the same as R.sup.3 or is .paren open-st.R.sup.5 --O.paren close-st..sub.x R.sup.5 ; R.sup.5 is the same as R.sup.3 ; PA1 m and n in formula (I) are integers which together are of sufficient value to achieve an R.sup.1 molecular weight of 300 to 30,000; m and n in formula (II) are integers which together are of sufficient value to achieve an R.sup.7 molecular weight of 300 to 30,000; and M represents hydrogen, an alkali metal, ammonium or P.sup.+ (C.sub.6 H.sub.5).sub.3 CH.sub.3, and PA1 (ii) a complex of a ferric halide selected from the group consisting of ferric fluoride, ferric chloride and ferric bromide and ethylene glycol or an oligoethylene glycol selected from the group consisting of di-, tri-, and tetraethylene glycol
Further, Wilson, et al, in U.S. Pat. No. 5,212,032, disclose, as coating materials for biasable transfer members, certain elastomeric polyurethanes containing, as conductivity control agents or charge control agents for controlling the resistivity of the elastomeric coating and hence that of the biasable transfer member to a range from about 10.sup.7 to about 5.times.10.sup.10 ohm cm, certain ionizable ferric halides selected from the group consisting of ferric fluoride, ferric chloride and ferric bromide complexed with ethylene glycol or an oligoethylene glycol selected from the group consisting of di-, tri-, and tetraethylene glycol.
In general, it has been found that for optimal toner image transfer to take place, that is, where all, or substantially all of the toner particles transfer from the surface of the photoconductor to the final support surface, that the polyurethane coating materials should possess a resistivity (i.e., a volume resistivity) of from about 10.sup.6 to about 5.0.times.10.sup.11 ohm cm. Volume resistivity, as defined herein, is the product of the applied voltage and the electrode surface area divided by the product of the sample thickness and the measured current. That is: ##EQU1## where .rho.=volume resistivity; V=applied voltage;
i=measured current, and PA2 t=sample thickness.
However, although the polyurethane materials of Chen et al and Wilson et al possess volume resistivities in a range compatible with or critical to optimal toner image transfer, they are deficient in that they both exhibit or possess relatively short electrical lives. That is, typically after about forty-eight hours of continuous use in an electrostatographic copying device, a biasable transfer member utilizing a polyurethane material of either Chen, et al or Wilson, et al must be removed from the copying device or machine and replaced with a new biasable transfer member because the original biasable transfer member no longer is capable of transferring a complete toner image from the photoconductor to the final support material (e.g. a sheet of paper). This is believed to be due to the following phenomena. Under normal operating conditions, it is necessary in order to achieve optimal image transfer to maintain a relatively constant current flow of less than about 30 micro amps in the nip area between the transfer roll surface, the transfer material and the photoconductive surface from which a developed image is to be transferred. For this condition to exist, the resistivity of the polyurethane material must be within critical values, i.e., from about 10.sup.6 to about 5.0.times.10.sup.11 ohm cm, as previously mentioned, and must be relatively constant under normally anticipated extremes of operating conditions. The electrical life, and hence the functional life of the biasable transfer member (i.e., the working life of the biasable transfer member) is directly related to the maintenance of this constant controlled resistivity region. That is, the electrical life of the biasable transfer member is largely determined by the stability of the output current and/or voltage versus time. (Bias roll power supplies are generally constant current or constant voltage devices with upper current or voltage limits which respond to changes in the resistivity of the biasable roll material, i.e., the polyurethane). Thus, as used herein, the term "electrical life" refers to a controlled, i.e., constant resistivity with time under an applied electrical field. Changes in the resistivity of the polyurethane material versus time are reflected in voltage demands required to maintain the constant current output of the material of which the device is made. As transfer current flows through the biased transfer member or roll, however, over time the ionic charge control or conductivity control additives in the polyurethane materials used in the biasable transfer roll migrate depleting ions and increasing the resistivity of the material causing the bias voltage to increase while maintaining a constant transfer current. Eventually, substantially all of the ions are depleted and the upper voltage limit is reached beyond which point the efficient transfer of toner can no longer take place resulting in incomplete toner transfer causing undesirable side effects such as mottle or no toner transfer at all. Thus, the material used in the fabrication of a typical biasable transfer member (e.g., a biasable transfer roll) has an intrinsic electrical life directly related to the ionic depletion of the conductivity control agent in the polyurethane material. Stated another way, the problem associated with bias roll transfer systems is that the electrical life of the bias transfer member is inversely proportional to the transfer current therethrough.
We have found, however, that by blending a dicarboxylate salt of Chen et al with a ferric halide/ethylene glycol or oligoethylene glycol complex of Wilson et al in a molar ratio of dicarboxylate salt to complex of 1.0:5.0 to 5.0:1.0 and incorporating the blend into a polymeric material such as a polyurethane material, that the blend not only provides a resistivity to the polymeric material of from about 10.sup.6 to about 5.0.times.10.sup.11 ohm cm, which is consistent with optimal toner image transfer but, in addition, improves or extends the electrical life and hence the functional life of the polyurethane material beyond the electrical life and the functional life of either of the polyurethane materials of Chen et al or Wilson et al.
Further, in addition to controlling the resistivity of the polyurethane material and extending or improving the electrical life thereof, it also has been found that incorporation of a blend of these particular materials in the amounts disclosed into a polyurethane material reduces the sensitivity of the resistivity of the material to changes in relative humidity.
The terms "biasable transfer member," "biased transfer member" or "bias transfer member," as used herein, refer to a member for electrically cooperating with a conductive support surface to attract electrically charged particles from the support surface towards the member.